The invention relates generally to the field of hydrofoils. In particular, the invention relates to a hydrofoil section having a "fish" shaped cross-sectional thickness and a trailing edge ventilated with a gas at or near the free stream ambient pressure.
Fundamentally, hydrofoils differ from aerofoils in that two fluid phases are possible across a hydrofoil. The two phases include a liquid phase and a gas phase. The liquid phase is water and the gas phase is water vapor or air, either separately or in combination. When the gas phase present is predominately water vapor, the hydrofoil is cavitating. When the gas phase present is predominately air, the hydrofoil is said to be ventilating. If no gas phase is present, the hydrofoil is referred to as subcavitating.
Cavitation and ventilation both appear as bubbles attached to the surface of the operating hydrofoil. This phenomenon particularly occurs over the section back (suction side) of the hydrofoil with the bubbles varying both as to size and extent. The formation of vapor bubbles will occur within a liquid in a region where the static pressure of the liquid's flow field is equal to, or less than, the saturation (vapor) pressure of the liquid. The resulting low pressure is a consequence of the local acceleration of the liquid to a relatively high velocity over the hydrofoil surface.
In order for cavitation to develop, the surface pressures on the suction side of the hydrofoil must be lower than water vapor pressure. Ventilation will develop when surface pressures exist which are lower than the ambient pressure of an externally available gas supply. The gas supplied is usually air from an atmospheric source. Although other sources may be employed.
Cavitation and ventilation are both ordinarily undesirable. While both cavitation and ventilation increase the section drag of the hydrofoil, cavitation is also barometrically unstable and can lead to problems such as vibration, excessive noise and erosion of the hydrofoil surface.
Whenever possible in the designing of hydrofoils, an attempt is made to avoid the occurrence of both cavitation and ventilation. In designing a hydrofoil for high speed applications, the development of cavitation and/or ventilation becomes increasingly difficult to avoid and if complete subcavitating conditions are insisted upon, the result is the sacrifice of low drag for adequate strength. For example, the achievement of complete subcavitating conditions in the hydrofoil sections of a planing boat propeller, having both reasonable efficiency and adequate strength, is generally impossible.
Minimum drag hydrofoil sections are often erroneously generalized as being subcavitating. In order to achieve subcavitating conditions across the hydrofoil section, measures are often taken which result in drag coefficients higher than those achieved if cavitation or ventilation was selectively designed into the hydrofoil section.
Two components of hydrofoil section drag can be identified, viscous skin friction drag and pressure drag. Viscous skin friction drag is proportional to the product of section length and section speed squared, and is generally independent of section shape. Thus, for a given speed, the viscous skin friction component of section drag is directly proportional to the length of the hydrofoil section.
Pressure drag is generally a manifestation of boundary layer separation. At low to moderate speeds, the separated boundary layer encloses a separation "cavity" of low pressure liquid. In the high speed flows of relevance to the present invention, two occurrences are possible. First, the separation cavity may vaporize to form a super cavity of vapor gas. Second, the separation cavity may be vented with some other "high" pressure gas to form a ventilation cavity. Thus, in the present invention, the pressure drag is a drag associated with the gas cavity.
For a set hydrofoil section thickness, the pressure drag varies inversely with the section length, i.e., the pressure drag becomes larger as the section becomes "blunter". If the hydrofoil section length is increased too significantly in the interest of reducing the pressure drag associated with cavitation, the surface area of the section will become so great that viscous skin friction drag will become excessive. Accordingly, in high speed applications, as section length is increased the curve of drag versus sectional length will exhibit a local minimum away from the extremes. Under these conditions, some cavitation or ventilation almost always exists for the hydrofoil section length corresponding to the minimum total section drag.
In designing for minimum drag in practical high speed applications, selectively allowing some degree of cavitation, and/or ventilation, over the hydrofoil section is required. An example is the supercavitating (or back ventilated) hydrofoil. In this case, the suction side of the hydrofoil section is entirely enveloped in gas while the pressure face of the hydrofoil is fully wetted. For optimum performance at very high speeds, a supercavitating/hydrofoil section was previously necessary.
The present invention is similar in spirit to the concept of the supercavitating hydrofoil. Namely, some amount of cavitation (in this case ventilation) is selectively allowed for by design. Ventilation gas is exploited by the present invention to achieve an improved subcavitation performance over a broader range of high speed hydrofoil applications.
A basic characteristic of the present invention is a "fish" shaped cross-sectional thickness distribution. The thickness increases from the leading edge to a point near the midchord of the hydrofoil section. Afterward, the thickness decreases some amount before again increasing in a "fishtail" flare to a local maximum at a concave trailing edge. Thus, the cross-sectional area has a "fish" configuration. The trailing edge thickness may be more or less than the midchord thickness, depending upon the requisite design demands.
Another characteristic of the present invention is that the trailing edge or base is ventilated with gas at or near free stream ambient pressure. A sufficient quantity of gas must be available to develop a full vent cavity downstream of the trailing edge. With low pressure and boundary separation developing behind the trailing edge, the relatively high pressure gas (free stream ambient pressure) will draw naturally into the vent cavity along a low pressure path originating near the gas source. As a result, both the suction side and the pressure face of the hydrofoil section will be subcavitating at the design condition.
The base vented subcavitating hydrofoil section provides many distinct advantages over conventionally designed hydrofoil sections. Relative to a subcavitating hydrofoil section of the same length and midchord thickness, the thicker "fishtail" flare and trailing edge increase the overall strength and stiffness of the base vented subcavitating hydrofoil section. Such an advantage is desirable in that most hydrofoils also serve as strength members and therefore must possess minimum cross-sectional dimensions in order to limit the effect of material stresses and deflections imposed by bending loads.
Another advantage of the present invention is that by ventilating the trailing edge, the strength increase is achieved with a very moderate increase in the overall section drag. While a relative subcavitating hydrofoil section of the same length and midchord thickness has negligible pressure drag associated with it, the base vented subcavitating hydrofoil section displays a minor pressure drag increase associated with the vent cavity. The amount of pressure drag increase will depend on the thickness gradient employed in each particular design of the "fishtail" flare. However, this pressure drag increase will be very small as compared to that which would occur if the boundary separation of the base was allowed to vapor cavitate, the natural occurrence prevented by the presence of the high pressure (atmospheric) gas supply.
The "fishtail" flare also prevents ventilation of the hydrofoil section back. The "fishtail" essentially develops high pressure fences along the trailing edge on both the section back (suction side) and section face (pressure side). Ventilation gas is thereby prevented from flooding low pressure liquid regions forwardly located on the section sides. Thus, except for the ventilation of the trailing edge, the hydrofoil section is fully wetted and subcavitating.
The "fishtail" further allows the hydrofoil section to reach higher operational speeds before vapor cavitation appears on the section back. This is accomplished because the increased thickness of the "fishtail" flare retards fluid flow velocities over the forechord of the hydrofoil section. Employing the present invention, a hydrofoil section can be designed to subcavitate at operational speeds considerably higher than a conventional aerofoil-type subcavitating section having the same length and midchord thickness. This is an important advantage of the present invention.
When incorporated into a lifting hydrofoil, the "fishtail" flare of the present invention has no effect on lift until back ventilation occurs. Upon the hydrofoil section being loaded to a level beyond its design lift, a point at which subcavitation still exists on the section face and back, the high pressure fence of the suction side trailing edge begins to breakdown and allow the suction side of the hydrofoil section to be flooded with ventilation gas. However, once back ventilation of the hydrofoil section has occurred, the designed lift development of the hydrofoil section is merely shifted from the meanline camber of the section to the camber line of the pressure face. At this load point and beyond, the face-side trailing edge of the "fishtail" operates as a trailing edge face camber, thereby "cupping" the fluid to maintain efficient lift in conjunction with low drag during back ventilated operation of the hydrofoil section.
The overall result of the present invention is a hydrofoil section which, over an increased range of operating conditions, maintains high efficiency and low drag.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and appended claims, taken in conjunction with the accompanying drawings.